1. Field of the Invention
This invention relates to steam re-compression vacuum dryers used for dehydrating and drying water-containing wastes, such as rubbish, waste fish, mud, etc. and also used for concentrating liquid wastes. More particularly, the present invention relates to a steam re-compression vacuum dryer, which comprises a vacuum tank for accommodating a waste, a compressing means for compressing steam from the vacuum tank, a steam condensing means thermally coupled thereto, and exhausting means for exhausting condensation water and non-condensing gas.
2. Description of the Prior Art
Wastes containing great amounts of water, such as organic wastes and mud, which could not heretofore have been burned and are readily spoiled, require a great deal of labor for their disposal. In addition, liquid wastes produced as a result of plating, photography, etc., are discharged in the ocean because their disposal is difficult. However, there is a trend for prohibiting such discharging in our country and also internationally.
Combustible wastes with even great water contents, however, can be burned with a slight amount of combustion promoter when their water content is reduced by evaporation. In addition, self-burning wastes can be used as a source of energy. Organic wastes can be readily converted into composts. Liquid wastes permit transportation cost reduction and are capable of recovering resources.
The evaporation of water content, however, requires application of heat corresponding to the high latent heat of water evaporation (2,257 kJ/kg at 100.degree. C.), thus requiring enormous heat energy.
Heretofore, techniques of drying water-containing wastes by vacuum evacuation have been used. In these techniques, a gas-tight vessel accommodating a waste is evacuated using a vacuum pump and, at the same time, heated with hot water or steam from a boiler, thus evaporating the water content. Steam being generated in this way is condensed by cooling water from a cooling tower and drained by using a water sealed vacuum pump.
By causing pressure reduction to below the atmospheric pressure, the boiling point of water is reduced from 100.degree. C. Therefore, the waste can be quickly dried while preventing a bad smell dispersion. In addition, wastes containing combustible materials such as alcohols can be safely dried.
However, the latent heat of the water evaporation is not reduced by evacuation. Rather, a boiling point reduction slightly increases the latent heat of evaporation (for instance to 2,383 kJ/kg at 50.degree. C.). Therefore, the heat energy required for drying cannot be reduced by evacuation.
As techniques for reducing heat energy required for evaporation, those of multiple stage flashing and steam re-compression are well known in the art. In either case, steam generated by evaporation is compressed and condensed (i.e., liquified) in a heat exchanger, and the latent heat of condensation generated at this time is used to evaporate water. Theoretically, these processes permit high energy efficiency to be obtained.
Particularly, since the steam re-compression process uses a relatively simple construction and permits efficient operation by electric power, it is used for concentrating liquid wastes. As the equipment, however, a large scale apparatus (of several 10 kW or above) is mainly used, which uses a centrifugal compressor. In addition, auxiliary devices such as a vacuum pump for air extraction and a draining pump are necessary, thus, complicating the equipment and increasing the cost thereof.
In applications, the process is used mostly for concentrating liquid wastes.
As a further technique, a vacuum evaporator, which is based on the principle of the heat pump and satisfactory in energy efficiency, is used in practice. In this case, a gas-tight vessel accommodating a waste is evacuated by using a water ejector or a vacuum pump, latent heat of condensation of steam is absorbed (with a refrigerant absorber) by using a heat pump based on such coolant as "HCFC-22" (chlorofluoro hydrocarbon), and latent heat of evaporation is given to the water (with the refrigerant condenser).
An advantage of this technique is that existing refrigerant (heat pump) parts can be used. However, it is possible that the lubricating oil will thermally be decomposed. Therefore, the cooling temperature cannot be made so high (only up to 60.degree. C. to 70.degree. C.). In addition, a heat exchanger between water and coolant is necessary, and the estimated coefficient of performance (which is the quotient of division of applied heat by electric input) is 3 to 5. The steam re-compression process permits a high heating temperature to be obtained and also permits a coefficient of performance of 10 or above to be readily obtained.
An object of the invention is to provide a steam re-compression vacuum dryer, which is based on the principle of steam re-compression, is small in size, is convenient to handle and is highly efficient in energy efficiency.